In drilling a borehole, it may be advantageous to control the direction of drilling. For example, a typical borehole may include a substantially vertical section, a lateral section which may be slanted or substantially horizontal, and a curve section which connects the vertical section and the lateral section. Drilling a borehole while controlling the direction of drilling is referred to as directional drilling. Directional drilling involves orienting a drill bit in a direction (i.e., a toolface orientation) which facilitates drilling in a desired direction.
Drilling a borehole may be performed using various methods, including by rotary drilling, by non-rotary (i.e., “sliding”) drilling, and by a combination of rotary and non-rotary drilling. In rotary drilling, the drill string is typically rotated from a surface location in order to rotate a drill bit which is connected to the distal end of the drill string. In non-rotary drilling, the drill bit is not rotated with the drill string from the surface location, but is either rotated, reciprocated or otherwise driven by a downhole drilling motor which is connected within the drill string and is also connected with the drill bit. In a combination of rotary and non-rotary drilling, the drill bit may be rotated by rotating the drill string from the surface location and may be simultaneously driven by a downhole drilling motor.
Directional control during rotary drilling may be achieved with a rotary steerable drilling device which is capable of providing a toolface orientation. Directional control during non-rotary drilling may be achieved with a bent drilling motor or a bent sub which is capable of providing a toolface orientation.
Drilling rates are typically greater during rotary drilling than during directional drilling. As a result, directional drilling using a rotary steerable drilling device may potentially offer faster drilling performance than directional drilling using non-rotary drilling techniques. The continuous rotation and directional control provided by a rotary steerable drilling device may also result in a smoother borehole. Unfortunately, rotary steerable drilling devices tend to be expensive and relatively complex.
As a result, there remains a need for drilling methods which utilize both rotary drilling and non-rotary drilling in order to achieve directional control over a wellbore as it is drilled. There also remains a need for drilling methods in which transitions between rotary drilling and non-rotary drilling can be achieved relatively simply and quickly.